By far the most of the absorption materials currently in use, frequently also designated as superabsorbers, consist of slightly cross-linked polyacrylates and therefore only a small part, if any, is degradable (see e.g. Stegman et al., Waste Manage. Res. 11 (1993) 155).
In addition to the pure polyacrylates there are also polyacrylates grafted onto stairch (DE-A 26 12 846). However, the starch content of these products (up to 25%) is low. In the case of higher starch contents a distinct deterioration of the absorption properties is observed. Due to the polyacrylate content the biological degradability of these products is also low.
Likewise, up to approximately 25% of a polysaccharide which is water-soluble at least to a limited extent can be worked into a cross-linked polyacrylate superabsorber by introducing the polysaccharide into the reaction mixture during the polymerization of the acrylate (DE-A 40 29 591, DE-A 40 29 592, DE-A-20 29 593).
U.S. Pat. No. 5,079,354 describes an absorber material based on carboxymethyl starch, that is, a starch ether, which is produced by reacting starch with chloroacetic acid. In this process an equivalent amount of sodium chloride relative to the chloroacetic acid used is released, which is undesirable for ecological reasons. In addition, it is known, that etherified polysaccharides are only poorly biodegradable at high degrees of substitution (Mehltretter et al., J. Am. Oil Chem. Soc. 47 (1970) 522).
DE-A 0,603,837 describes the production of starch esters using organic acid anhydrides. To this end starch of diverse origins is allowed to react in a one-stage aqueous process with organic acid anhydrides of general formula I ##STR1## in which R signifies alkyl, aryl, alkenyl, alkaryl or aralkyl with 1 to 7 C atoms under certain conditions of pH, temperature and concentration. Starch esters enumerated by way of example in the specification of EP-A 0,603,837 include starch acetate, starch propionate, starch butyrate, starch hexanoate, starch benzoate or also mixed starch acetates/propionates. In the examples of EP-A 0,603,837 the use of propionic acid anhydride, acetic anhydride and/or butyric acid anhydride is disclosed. The problems which otherwise resulted when using rather large amounts of anhydride such as e.g. the swelling or gelatinization of the starch and problems during separation of the starch esters from the reaction mixture are successfully avoided with the process described in EP-A 0,603,837. The product becomes hydrophobic as a result of the reaction and can therefore be filtered off in a simple manner.
WO 93/01217 (PCT/EP 92/01553) teaches a method of producing starch esters for clinical, especially parenteral application. The starch esters according to WO 93/01217 are quite water-soluble, which is necessary for parenteral application.
There have also already been attempts to create biodegradable superabsorbers. Thus, DE-A 42 06 857 teaches an absorption means consisting of a component based on special, renewable polysaccharide raw materials, of a special, water-swellable polymer, of a matrix material, of an ionic or covalent cross-linking agent and of a reactive addition. The component based on renewable polysaccharide raw materials comprises e.g. guar, carboxymethyl guar, xanthan gum, alginates, gum arabic, hydroxyethyl cellulose or hydroxypropyl cellulose, carboxymethyl cellulose and other cellulose derivatives, starch and starch derivatives such as carboxymethyl starch. It is furthermore known from DE-A 42 06 857 that the cited polymers can be modified by cross-linking in order to reduce their water solubility and to achieve better swelling properties. The cross-linking can take place in the entire polymer or only on the surface of the individual polymer particles.
The conversion of the polymers can take place with ionic cross-linking agents such as e.g. calcium compounds, aluminum compounds, zirconium and iron(III) compounds. Likewise, a conversion is possible with polyfunctional carboxylic acids such as citric acid, mucic acid, tartaric acid, malic acid, malonic acid, succinic acid, glutaric acid, adipic acid, with alcohols such as polyethylene glycols, glycerol, pentaerythrit, propane diols, sucrose, with carbonic acid esters such as glycoldiglycidyl ether, glycol di- or triglycidyl ether and epichlorohydrin, with acid anhydrides such as succinic acid anhydride and maleic acid anhydride, with aldehydes and multifunctional olefins such as bis-(acrylamido) acetic acid and methylene bisacrylamide. Of course, derivatives of the families cited can also be considered as well as heterofunctional compounds with different functional groups of the families cited above.
Although the systems based on sodium carboxymethyl cellulose presented and documented with examples have quite favorable absorption properties in combination with sodium polyacrylate in various tests, no particulars about the biodegradability can be gathered from the publication.
However, it is known that polyacrylates are substantially not biodegradable (R. Stegmann et al., Waste Water Res. 11(2) (1993) p. 155) and carboxymethyl cellulose, a polysaccharide ether, is only very poorly biodegradable (4.6% after 5 days; M. Seekamp, Textilveredlung, Vol. 25 (1990) p. 125.
An absorption material with a biodegradability which is considerably improved in comparison to the polyacrylates primarily used at the time is disclosed in EP-A 0,714,914, which discloses a swellable starch ester which consists of more than 50% by weight of non-water-soluble components and which has a retention capacity for 0.9% by weight aqueous NaCl solution of &gt;500% relative in each instance to the weight of the dry starch ester. The retention capacity is determined in that 0.1 g of the starch ester welded into a nylon bag with a mesh width of 52 .mu.m is allowed to swell for 30 min in 0.9% NaCl solution, the bag is subsequently centrifuged 5 min at 1400 rpm and then any resulting weight increase is gravimetrically determined.
Although the absorption materials presented in EP-A 0,714,914 and based essentially on maleic acid anhydride have very good swelling properties and good biodegradability the starch maleates of EP-A 0,714,914 unfortunately still have distinct disadvantages.
A distinct decrease of the swelling properties occurs after a few weeks. After several weeks very poor swelling properties are obtained both in the measurement of the retention capacity (SRV) and in the determination of the absorption capacity with and without load (AFK5 and A20FK5). This aging has sharply hindered the commercial use of starch maleates as absorption materials in the past.